1. Field of the Invention
The present invention relates to clutch mechanisms, and in particular to clutch mechanism usable for power screwdrivers.
2. Description of the Related Art
Hand-held power screwdrivers are known that can be held by a hand of an operator for performing a screw driving operation. In order to drive a screw into a workpiece, the operator sets the screw to a driver bit mounted to a front end of a spindle and presses the screwdriver toward the workpiece, so that the spindle retreats to connect a clutch mechanism, whereby the spindle rotates to drive the screw into the workpiece.
The clutch mechanism is provided between a driver side member and the spindle. The spindle is supported so as to be movable in an axial direction relative to the drive side member. In general, the clutch mechanism is a meshing clutch and includes drive-side clutch teeth and driven-side clutch teeth. The rotation is transmitted from the drive side member to the spindle when the drive-side and driven-side clutch teeth engage with each other according to the axial movement of the spindle. No rotation is transmitted when the drive-side and driven-side clutch teeth are disengaged from each other.
As the spindle advances according to the progress of the driving operation, the engagement between the drive-side clutch teeth and the driven-side clutch teeth becomes shallower. When the screw has been completely driven, the engagement between the drive-side clutch teeth and the driven-side clutch teeth is released, so that the rotation of the spindle is stopped.
In this kind of meshing clutch mechanisms, there has been known to incorporate a synchronization mechanism for reducing an impact that may be produced when the drive-side and driven side clutch teeth are brought to be engaged with each other. Techniques relating to the synchronization mechanism are disclosed, for example, in Japanese Laid-Open Patent Publication No. 2005-066782 (also published as Japanese Patent No. 4334944) and Japanese Laid-Open Patent Publication No. 2010-94773. According to the synchronization mechanisms of these publications, prior to the engagement of the driven-side clutch teeth with the drive-side clutch teeth by the retreating movement of the spindle, a part of the rotative force is transmitted to the spindle to cause preliminary synchronized rotation of the spindle. Therefore, the drive-side clutch teeth and the driven-side clutch teeth engage with each other in the state that a difference in the rotational speed between these clutch teeth has been reduced. Hence, the clutch mechanism does not produce a substantial impact when it is connected. Because the clutch mechanism is smoothly connected without producing a substantial impact, the durability of the clutch mechanism is improved and the operation of the power screwdriver can be smoothly performed.
However, in the case of the synchronization mechanism disclosed in the above Publication No. 2005-066782, a diametrically deformable member, a movable member (a steel ball) and a positioning member are interposed between the spindle and a drive gear (i.e., a drive side member) in the state that they always receive a biasing force of a spring not only at a stage of causing synchronized rotation of the spindle. Although the movable member is in a point-to-point contact relationship with the spindle, a part of the rotational force of the drive side is always transmitted to the spindle. Therefore, it is difficult to completely inhibit rotation of the spindle when the spindle is in an initial position before the retreating movement. In addition, it has been desired to further reduce wear of the components that constitute the synchronization mechanism.
In the case of the synchronization mechanism disclosed in the Publication No. 2010-94773, a metal synchronization member provided on the spindle is brought to slidably contact with the outer circumference of a drive gear immediately before engagement of the clutch teeth, so that the frictional force causes synchronized rotation of the spindle. Therefore, there is a problem of causing substantial wear of the outer circumference of the drive gear.
Therefore, there is a need in the art for a clutch mechanism usable for a power screwdriver and having an improved synchronization mechanism.